Closed-loop refrigeration/heat pump systems conventionally employ a compressor that is meant to draw in vaporous refrigerant at relatively low pressure and discharges hot refrigerant at relatively high pressure. The hot refrigerant is then cooled in a gas cooler if the pressure and temperature are higher than values of temperature and pressure at the critical point, otherwise it condenses into liquid, and the gas cooler is called a condensers, accordingly. “Critical point” is a physical property of pure substances defined by temperature and pressure. Above the critical point, the substance is in a supercritical state and comprises a supercritical fluid which is neither gas nor liquid.
Together with a compressor, an expansion device, which typically comprises an expansion valve, or in some cases may comprise one or plurality of capillary tube(s), divides the system into high and low pressure sides. The working fluid passes through the expansion device into an evaporator, and as it passes through the expansion device the fluid expands and cools. The fluid typically enters the evaporator in a liquid-rich state, and thereafter absorbs heat and evaporates. At low heat loads in certain working conditions it is not possible to evaporate all the liquid. Some amount of liquid refrigerant is used to dilute cycling oil and carry it back to the compressor. However, a large amount of liquid is undesirable because system efficiency could be lowered and the compressor could be significantly damaged if a large amount of liquid refrigerant enters the compressor (known as “liquid slugging”). Therefore, it is preferable to place an accumulator between the evaporator and the compressor to separate vapour and liquid and store the excess liquid. Accumulators have a metering function of collecting liquid and returning a certain amount to the compressor. This prevents liquid slugging and controls oil return. It is particularly important in automobile air conditioning systems, where surges of liquid refrigerant occur frequently because of the varying dynamic operating conditions. Moreover, use of an accumulator can elevate the efficiency of the evaporator in that dry coils, employed in traditionally operated evaporators, are not required.
Transcritical refrigerating systems operate in a range of temperature and pressure that cross the critical point of the refrigerant. In these systems, for refrigerants with relatively low critical temperatures, e.g. carbon dioxide which has a critical temperature of 31.7° C., it is difficult to reach a high specific cooling capacity and this is a significant barrier for achieving a high coefficient of performance (COP). To overcome this limitation, an internal heat exchanger is used that exchanges heat between refrigerants of different parts; one which connects the condenser/gas cooler and expansion device, and the other which connects the evaporator and compressor. This method is described in U.S. Pat. No. 5,245,833, 6,523,365 and 6,681,597.
Another feature of a known refrigeration system is the inclusion of the expander in the accumulator-heat exchanger system (U.S. Pat. Nos. 5,622,055 and 6,530,230). However, in U.S. Pat. No. 6,530,230, an expansion device is simply assembled at the inlet of accumulator-heat exchanger without being functionally integrated.
In U.S. Pat. No. 5,622,055, an expander, a heat exchanger, and an accumulator are integrated into a canister. However, those explorations focus only on subcritical refrigerants. The characteristics of trans-/hypercritical alternatives were not considered; accordingly, a phase change from supercritical fluid to liquid which occurs in trans-hypocritical systems was not taken into account in the expander design. Additionally, the capillary coils were required to be immersed in the liquid-phase of the accumulator. This will not increase the specific cooling capacity, and the extra circulating refrigerant needed will consume more energy. As a result, the whole system performance might not be improved significantly. Furthermore, the heat gain from environment between the evaporator outlet and the inlet of compressor (including accumulator) will decrease the system COP.